Solve Quartic with Casio FX-115MS

[Albert Chan]

(11-29-2018 05:28 PM)Albert Chan Wrote:  A trick to force Casio FX-115MS to solve for D: 0ABC + B^2 - 4(2D - A)(D^2 - C)

Discover a simpler way to solve for D, without the 0ABC + ... hack

B^2 - 4(2D - A)(D^2 - C) [SOLVE]

Enter the values like before, but scroll back to D= line before pressing [SOLVE]

If A, B, C were values already stored, scroll directly to D=, enter guess, scroll up, then [SOLVE]

If calculated D too small, below max(A/2, √C), press [SOLVE] again with higher guess.

This was vaguely (if followed the keystrokes) stated in the manual ... Oh well

Edit: although D can be solved with Cubic Solver, I worried about its bugs.
Calculated D must be very accurate, otherwise all 4 roots will be bad.

Without the bugs, Cubic Solver is easier. Just enter the coefficients:
a = -8, b = 4A, c = 8C, d = B^2-4AC, we get x1 = 1.99130646